Electric control circuit



Dec. 2s, 1943. G] ROGERS K 2,337,932

ELECTRIC CONTROL CIRCUIT Filed Oct. 5l, 1941 2 Sheets-Sheet 1 Inventor:

George L. Rogers,

y H1-@Attorney G. L. ROGERS lELECTRIC CONTROL CIRCUIT Filed Oct. 31, 1941 by WW 2 Sheets-Sheet 2 4wHr-:N sz, 34 AND as (ARE OMITTE D. ,l

WHEN 45,46 AND 49 {ARE- OMITTED.

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-I nventor: George L ,.Rogers,

H is Attorney.

Patented Dec. 28, 1943 ELECTRIC CONTROL CIRCUIT George L. Rogers, Schenectady, N. Y., assigner General Electric Company, a corporation of New York Application ctober 31, 1941, Serial No. 417,352 17 Claims. (Cl. 172-238) My invention relates to4 electric control circuits and more particularly to electric valve circuits for controlling the phase of an alternating voltage supplied to an output circuit.

In many electric control systems, it is desirable to employ an alternating voltage or a perlodic voltage oi adjustable phase relationship respect to the voltage of an associated supwith ply circuit. 0f course, alternating voltages which are adjustable in phase may he obtained by using' apparatus of the rotary type, such as that having electrically distributed stator and rotor characteristics. However, there are many instances in which apparatus of the rotating type or construction is undesirable from the standpoint of cost and construction. Heretofore, the phase shifting circuits of the static impedance type have not afforded the desired flexibility range oi the phase of the output voltage without involving an inordinate increase in manufacturlng costs. ln accordance with the teachings oi my invention, provide a new and improved phase shifting circuit which affords great exiloility ci control and which may loe constructed at reasonable cost.

It is an obiect ci my invention to provide a new and improved phase shifting circuit of the static impedance type..

It is another ohiect of rny invention to provide a new and improved phaseshifting circuit wherein the phase of the voltage applied to an output circuit may ce varied throughout a substantial angle, such as 36S electrical degrees.

it is a further object of my invention to provide a new and improved electric valve phase shifting circuit.

It is a still iurther object of my invention to provide new and improved control ,circuits for ties of the electric valves may be controlled se lectively in response to the magnitude of a unidirectional control voltage.

Briefly stated, in the illustrated embodiments of :my invention l have shown my invention as applied to a system for controlling the phase of the alternating voltage supplied to an output circuit from an associated alternating current supply circuit. The phase shifting circuit comprises a plurality of electrically displaced phase windings each having connected thereto variable impedance means such as a pair of reversely connected electric valves or electric discharge devices, the conductivities of which are selectively controlled to control the phase of the output voltage relative to the voltage of the supply circuit.

of control and A electric valve apparatus wherein the conductiviycharge devices shown in Fig. l.

I also provide a plurality of excitation circuits connected to the grids of the electric discharge devices, and which are selectively responsive to the magnitude oi a unidirectional voltage applied thereto in order 'to control selectively the conductivities of the respective pairs o discharge de-` vices. In this manner, the desired control or shift in phase of the output voltage to any de-I sired value may he readily obtained merely by controlling the magnitude of the unidirectional voltage applied to all of tht-,excitation circuits.

For a better understanding of my invention,

vreference may be had to the following description taken in connection with the accompanying drawings, and its scope will he pointed out in the appended claims. Fig. 1 of the drawings illustrates my invention as applied to a phase shifting arrangement for controlling the phase of the voltage applied to a single phase output or load circuit from a polyphase alternating current sup ply circuit. Fig. 2 represents an aspect of the control circuit shown in Fig. l. Fig. 3 represents certain operating characteristics relative to the control of the respective pairs of electric dis- Figs. 4, 6l and 8 represent in detail portions of the respective excitation circuits connected to the three groups of electric discharge devices shown in Fig. l; and Figs. 5, i and 9 represent certain operating characteristics of the control or excitation cir-x cuits shown in Figs. e, 6 and 8, respectively. Fig. l0 represents a complete arrangement wherein the keucitaticn circuits are shown connected to the phase shifting arrangementoi Fig. l.

Referring now to the arrangement shown in Fig. l, l have there illustrated my invention as applied to a phase shifting circuit for controlling the phase oi the voltage applied to an output circuit l which may include a transformer if.. 'lhe phase shifting'circuit may be energized from a polyphase alternating current supply circuit 2l and includes a plurality oi n electrically displaced phase windings. Although not limited thereto, I have illustrated my invention as applied to a system which is energized from a three phase al ternating current supply circuit and in which the electrically .displaced windings are provided by a transformer l having a pluralityof primary windings 5 and a plurality oi electrically displaced secondary or phase windings 6, i and having an electrically intermediate or neutral connection 9 which is connected to one terminal of the output circuit.

The phase shifting circuit also includes a plurality of variable impedance means such as n pairs of reversely connected electric discharge devices I0, II; I2, I3; I4, I5 which are connected to phase windings 6, I and 8, respectively. Electric discharge devices I Il-I5, inclusive, are preferably of the high vacuum type each including an anode I6, a cathode II and a control member or grid I8. The grids I8 of the discharge devices are connected to excitation circuits described in detail hereinafter and which are represented in Fig. 1 as comprising circuits I9, 20 and 2I. It

will be noted that the common .iunctures of the respective pairs of electric discharge devices are connected to one terminal of the load or output circuit I. s

I connect between the excitation circuits Il, 2l and 2| and the respective associated pairs of electric discharge `devices transformers 22, 23 and 24 to control the conductivities of these discharge devices so that the pairsoperate as an equivalent variable resistance in response to the variations in the grid voltages applied thereto. It will be noted that each of the discharge devices in the respective pairs is connected reversely, that is, having the anode of one connected to the cathode of the other, in order to transmit both half cycles of alternating current of the respective associated phase windings. Transformers 22-24 are preferably arranged to have a 1:1 ratio, thereby serving to maintain the grid and cathode of electric discharge devices I0, I2 and I4 at the same potential so that only the unidirectional excitation is effective. In this manner, the alternating component of voltage due to theconnection to the ano-de-cathode circuit is effectively neutralized so far as each of the grid control or excitaticn circuits is concerned. Fig. 2 represents a portion of the circuit shown ln Fig. 1 in which the aspects of the neutralization, by using the transformer connections of this nature, are more succinctly represented. It will be noted that the secondary winding 25 is connected across the grids I8 of electric discharge devices I0 and II, and that the primary winding 28 is connected to the common junctures of the cathodes of the discharge devices. In this manner, the alternating components of voltage which are present in the anode-cathode circuits of the pairs of discharge devices is effectively eliminated from the grid circuits so that the discharge devices respond precisely to the magnitude of the unidirectional voltages of the control circuits. Certain features of the circuit just described for neutralizing the alternating-current component of voltage in the excitation circuit of the electric valves are claimed in my divisional application Serial No. 482,986, filed April 14, 1943, and assigned to the assignee of the present application.

The control circuits or excitation circuits I9- 2I which I provide for selectively controlling the conductivities of the discharge devices I Il-I5, inclusive, include means, specifically described hereinafter, which are selectively responsive to a unidirectional 'control voltage and selectively control the conductivities of the different pairs in different manners in response to different magnitudes of the v.unidirectional control voltage. I provide this type of control in order to vary lor control the phase of the output voltage supplied to output circuit I. For example, yI have found that if the conductivity of one of the pairs of electric discharge devices, such as discharge devices I and II, is maintained substantially constant or at a zero value, the phase of the voltage supplied to the throughout output circuit I may be varied electrical degrees and, lastly, by choosing another combination of discharge devices controlled in a corresponding manner, the shift in phase may be made throughout another electrical degree interval thereby completing the ypressing on the grids I8 phase shift throughout the complete 21|- or 360 electrical degrees.

The diagram of Fig. 3 may be of assistance in explaining the manner in which the respective pairs of electric discharge devices shown in Fig. 1 may be controlled to obtain any desired shift in phase of the voltage supplied to output circuit I. If the voltages of the excitation circuits I9, 2U and 2l be represented by the characters Ex, Ey and Ez, respectively, and if it be assumed that the electric discharge devices are of the type which are rendered completely nonconducting by imthereof a -5 volts, the relation of the shift in phase and the respective grid voltages may be represented by the curves shown in Fig. 3. By referring to these curves, it will be noted that during any one interval of time the conductivity of one pair of discharge devices is reduced to zero by impressing thereon a negative hold-oir voltage of 5 volts and by concomitantly varying the conductivities of the other two pairs of electric discharge devices.

Figs. 4, 6 and 8 represent the excitation circuits which are connected to circuits I9, 20 and 2|, respectively, of Fig. 1. These excitation circuits are energized from a common control circuit including a positive conductor 21 and a negative conductor 28, and the voltage of which is represented by the character E1. These circuits are selectively responsive to the magnitude of the unidirectional voltage E1 and control the maghi` tude of the bias voltages impressed on the grids I8 of the respective associated pairs of electric discharge devices.

Considering Fig. 4, I there provide an arrangement in which the voltage applied to circuit I9 varies in the manner indicated by the solid curve A of Fig. 5. It will be noted that the bias voltage Ex increases to a negative maximum value upon increase of voltage E1 to a predetermined value, and that the bias voltage remains substantially constant through a second region or range until the voltage E2 attains a second predetermined value,'and that the bias voltage progressively decreases in magnitude upon further increase of E1 beyond the second predetermined value.

In order to obtain this type of control, I provide in Fig. 4 a circuit including a voltage divider comprising a pair of serially connected resistances 29 and 30 having a common juncture 3| which is connected to one conductor of .circuit I9.

maintain the voltage of 'circuit 33 in series relation connected between the nega.

tive Iconductor 28 and one terminal of circuit I8. As the voltage E1 increases, a proportionate voltage is transmitted to circuit i9. The component of voltage so transmitted is determined, of course,

by the ratio of resistanceszs and 3U. I have found that these resistances may, if desired, be

made equal so that one-half the voltage E1 is 5 or the circuit I9. To I9 substantially constant for the second range of voltage E1, I employ a source of reference voltage, suchas a battery 34 and a unidirectional conducting device 35, which are oppositely poled and connected to transmit current i when the voltage appearing across resistance 30 attains a value corresponding to the first predetermined value of E1. Throughout the second range of E1, the unidirectional conducting device 35 transmits current throughA resistances 3G and 32 to maintain the voltage transmitted to circuit i9 at a substantially constant value. I provide a second source of reference voltage, such as a battery 36 and a unidirectional conducting device 3i, which are oppositely poled and connected in series relation between the positive conductor 2l and the resistance to decrease progressively the magnitude of the bias voltage when E1 increases in magnitude beyond the second preden termined value.

It will be understood that the operating characteristics shown in Fig. 5 represent the theoretiavailable for the control cal shape of the curve which would be obtained by assuming that no voltage appears across the unidirectional conducting devices. However, inasmuch as the devices necessitate the impression of a predetermined voltage thereacross, it will be appreciated that the curves actually when plotted from observed data will have somewhat rounded portions instead of the sharp characteristics shown. The upper dotted portion of the characteristics shown in Fig. 5 may be obtained and the characteristics changed by omitting the source of reference voltage or battery 3ft, resistance and device 35.

The operation oi the arrangement sho-wn in Fig. l will be explained by considering the circuit upon a progressive increase in magnitude of E1. During the first range of E1, the voltage applied to circuit i@ increases progressively and linearly due to the rise in voltage across resistance 3U. Upon attaining a predetermined value and for voltages within a predetermined range above that value, the voltage drop across resistance it@ is sufficient to overcome the voltage of battery Ml, causing current to flow in a circuit including batterif M, device 35, resistance 530 and resistance t2. The voltage drop appearing across resistance 32 is sumcient to neutralize or counteract the rise in voltage appearing across resistance occasioned by increase of voltage E1, thereby maintaining the voltage supplied to circuit it substantially con stant throughout a second range of E1. When the voltage of E1 increases to a second predeterm mined value, causing unidirectional conducting device 3l to conduct current, the magnitude of the voltage transmitted to circuit ia is progressively decreased. During this region or operation current is conducted by battery at, device 3l, and resistances 33 and 32. Upon a decrease of E1, curve A is traversed in the reverse direction.

through resistances 30 and 32 dii - Control circuit 20, shown in detail in Fig. 6, is constructed to produce a voltage having the characteristic of curve B in Fig. '7. In this arrangement I provide a. source of voltage, such as a battery 38 and resistances 39 and 40 which transmit a voltage of predetermined value to circuit 20 when the value of E1 is zero and which progressively decreases the magnitude of the voltage supplied to circuit 2 for increase of E1 within the iirst range of E1 by the transmission oi current through resistances 39 and 40. When E1 attains the predetermined value, the voltage supplied to circuit 20 is reduced to zero and upon a further increase of voltage of E1 into the second range I provide va unidirectional conducting device 4I and a source of reference potential, such as a battery 42, which conduct greater amounts of current through resistance 39 to increase pron gressively the magnitude of the bias voltage Ey until the voltage E1 attains a second predetermined value. As a means for limiting the magnitude of the voltage supplied to circuit 2@ upon increase of E1 beyond the second predetermined value, I connect across circuit 20 a unidirectional conducting device 43 and a source of reference voltage such as a battery M.

The voltage Ey may be made to have the characteristic shown by the dotted line Within the third region or range of operation by omitting resistance liti, unidirectional conducting device t3 and the source of voltage Mi.

The circuit of Fig. 8 produces an output characteristic corresponding to curve C of Fig, 9. In this arrangement the voltage applied to circuit 2i is maintained substantially constant throughout the iirst range of E1 by means oi a source of voltage i5 and resistances 46 and lil. When E1 at tains a predetermined value, the magnitude of the bias voltage supplied to circuit 2i is progressively decreased throughout f a second predetermined range of E1 by means of a source of reference voltage it and unidirectional conducting device ia which complete a circuit through resistance iii and source d5. When E1 attains a second predetermined value, the voltage supplied to circuit 2i is then progressively increased in magnitude by the provision of a source of reference voltage 5G and a unidirectional conducting device 5i which are connected in the manner shown from one tern minal of circuit 2i to the negative conductor at. rlhroughout this third mentioned range when the voltage E1 attains or exceeds the second predetermined value, the voltage supplied to the circuit 2i is progressively increased due to the iiow of current through device 5i and source 5B.

The voltage Ez may be made to have the characteristic indicated by the dotted line Within the iirst range of voltage E1 by omitting source @5, resistance [it and unidirectional conducting device da.

Although not limited to the particular range of applied voltages and output voltages, I have round that the circuits shown in Figs. Ll, 6 and 8 operate very satisfactorily when the respective elements have the following values:

E1=range from zero to 3G volts E2=l5 volts Resistance 29=resistance 30 ties of discharge devices I4 variation in conductivitles, represented by the curves of Fig. 3, in response to variations in magnitude of the voltage E1. As the voltage E1 progressively increases from zero to the first predetermined value, that is in the first range, discharge devices I4 and I5 are maintained 'nonconducting and the conductivities of electric discharge devices I2 and I3 are progressively increased and the conductivities of discharge devices I and I I are decreased, thereby shifting the voltage applied to the output circuit I through 120 electrical degrees. VThroughout the second range ofvoltage E1, the discharge devices I0 and II are maintained non-conducting, and the conductivities of electric discharge devices I2 and I3 are progressively decreased, and the conductiviand I are progressively increased. Throughout the third range, discharge devices I2 and I3 are maintained nonconducting and the conductivities of devices I4 and I5 are progressively decreased and the con-A ductivities of discharge devices I0 and l I are progressivelyincreased to provide an additional 120 degrees. shift in phase. In this manner, the phase of the voltage impressed on the 'output circuit may be smoothly controlled throughout three consecutive or adjacent 120 electrical degree intervals, and the phase displacement of the output voltage may be accurately controlled or determined by the magnitude of the voltage El which is impressed on all three of the excitation circuits. Upon decrease in the magnitude of voltage E1, the output voltage is shifted in the opposite direction. Certain features embodied in the control voltage producing circuits illustrated in Figs. 4, 6, and 8 are claimedin my divisional application Serial No. 482,985, filed April 14, 1943, and assigned to the assignee of the present application.

While I have shown and described my invention as applied to a particular system of connections and as embodying various devices diagrammatically shown, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

.What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, an alternating current supply circuit, an output circuit, means connected between said supply circuit and said output circuit comprising a plurality of electrically displaced phase windings connected for energization from said supply circuit and having an intermediate connection connected to said output circuit, a plurality of variable impedance means each connected to a different one of said windings and having a common juncture connected to said output circuit, and means for controlling said variable impedance means to elect the selective variation ofthe impedance of the means connected with the different terminals and thereby to control the phase of the voltage applied to said output circuit with respect to the phase of the voltage of said supply circuit.

2. In combination, an alternating current supply circuit, an output circuit, means connected between said supply circuit and said output circuit comprising a plurality of electrically displacedphase windings connected to be energized from said alternating-current supply circuit and having an intermediate connection connected to said output circuit, a plurality of variable impedance means each connected to a diierent one of said windings and having a common juncture connected to said output circuit, and means for selectively controlling the impedances of said variable impedance means to control the phase of the voltage applied to said output circuit rwith respect to the phase of the voltage of said supply circuit.

3. In combination, an alternating current supply circuit, an output circuit, means connected between said supply circuit and said output circuit comprising a plurality of electrically displaced phase windings connected to be energized from said alternating-current supply circuit and having an intermediate connection connected to said output circuit, a plurality of variable impedance means each connected to a different one of said windings and having a common juncture connected to said output circuit, a plurality of control circuits connected to said variable impedance means and each comprising means selectively responsive to the magnitude of the voltage supplied thereto, and a circuit connected to said control circuits for applying thereto a variable voltage to effect selective control of the impedances of said variable impedance means to control the phase of the voltage supplied to said output circuit with respect to the phase of the volt age of said alternating-current supply circuit.

4. In combination, an alternating current suphaving an intermediate connection connected to said output circuit, a plurality of pairs of reversely connected electric discharge devices of the high vacuum type, each pair of said discharge devices being connected to a different one of said phase windings and each discharge device including a grid for controlling the conductivity thereof, said pairs of electric discharge devices having a common connection to said load circuit, and a plurality of excitation circuits each connected to the voltage transmitted to said output circuit with respect to the phase of the voltage of said alternating-current supply circuit.

5. In combination, an alternating current supply circuit, an alternating current output circuit, means connected between said supply' circuit and said output circuit and comprising a plurality of electrically displaced phase windings connected to be energized from said supply circuit and having an intermediate connection connected to said output circuit, a plurality of pairs of reversely connected electric discharge devices, each pair of said discharge devices being connected to a diliei ent one of said phase windings and each discircuits including means selectively responsive to the magnitude of the voltage applied thereto, and means for supplying to said excitation circuit a asa-:,932

said load circuit with y 6. In combination, an alternating current'supl ply circuit, an output circuit, means connected between said supply circuit and said-output circuit for controlling the phase of the voltage supplied to said output circuit withrespect to the phase of the voltage of said alternating-current supply circuit comprising a plurality of electrically displaced phase windingsconnected to be energized from said supply circuit and having anl intermediate connection connected to said output circuit, a plurality oi electric discharge devices, each of said discharge devices being connected to a different one of said phase windings and each-- discharge device including a grid for controlling the conductivity thereof, and excitation means connected to the grids for selectively increasing the conductivity of one discharge device and decreasing the conductivity of another discharge device.

'7. In combination, an alternating current supply circuit, an output circuit, means connected between said supply circuit and said outpllt circuit for controlling the phase of the voltage supplied to said output circuit with respect to the phase of the voltage of said supply circuitcomprising a plurality of electrically displaced phase windings connected to be energized from said supply circuit and having an intermediate connection connected to said output circuit, a plurality of pairs of reversely co-nnected electric dischargerdevices, each pair of said discharge devices beingr connected to a different one oi said phase windings and each discharge device including a gridl for controlling the conductivity thereof, a control circuit for producing a voltage of variable magnitude, and a plurality of excitation circuits connected between said r:ontrol circuit and the grids of said discharge devices and including means selectively responsive to the magnitude of said voltage for selectively increasing and decreasing the conductivities of predetermined diiIerent pairs of discharge devices.

8. In combination', an alternating current supply circuit, an output circuit, means connected between said supply circuit and said output circuit for controlling the phase of the voltage supplied to said output circuit with respect to the phase of the voltage of said supply circuit comprising a plurality of electrically displaced phaseV windings connected to be energized from said supply circuit and having an intermediate connectionconnected to said output circuit, a plurality of pairs of reversely connected electric discharge devices, each pair of said discharge devices being connected to a different one of said phase windings and each discharge device including a grid for controlling the conductivity thereof, a control circuit for producing a control voltage of variable magnitude, and a plurality of excitation circuits connected between said control circuit and said grids for selectively Vcontrolling the conductivities of the respective pairs of electric discharge devices in response to the magnitude of said control voltage.-

9. In combination, a, three phase alternating current supply circuit, an output circuit, means connected between said supply circuit and said selectively controlling output circuit for controlling the phase of the voltage supplied to said output circuit with respect to the phase o! the voltage of said supply lcircuit comprising three electrically Adisplaced phase windings energized from said supply circuit and having an intermediate connection connected to said output circuit, three pairs of reversely connected electric discharge devices, each pair oi said discharge devices being connected to a dierent one of said phase windings and each discharge device including a grid for controlling the conductivity thereof, said pairs of discharge devices having a common connection to said load circuit, a control circuit for producing a variable control voltage. and a plurality of excitation circuits connected to saidl grids and said control circuit for selectively increasing and decreasing the conductivities of the respective pairs of discharge devices, the excitation circuits comprising means for maintaining the conductivity of one pair of electric discharge devices substantially zero during a predetermined range of variation of said control voltage and for increasing the conductivity of a second pair and decreasing the conductivity of a third pair during the same range of said control voltage whereby the phase of the voltage supplied to said output circuit is shifted throughout substantially 120 electrical degrees.

circuit for controlling the phase of the voltage supplied to said output circuit with respect to the .phase of the voltage of said supply circuit and comprising n electrically displaced phase windings connected to be energized from said supply 'circuit and having an intermediate Lconnection connected to said output circuit. a plurality of pairs of reversely connected electric discharge devices having common junctures connected to said output circuit, each of said pairs of said discharge devices being connected to a different one of said phase windings and each discharge device including a grid for controlling the conductivity thereof, a plurality of excitation circuits each connected to a different pair of electric discharge devices and selectively responsive to the magnitude o! the voltage applied thereto, and a control circultconnected to said excitation circuits for varying the magnitude of the voltage applied to said excitation circuits and for the conductivities of different combinations of said discharge devices to shift the phase of the voltage applied to said output circuit.

v rality of pairs ofreversely connected electric discharge devices having common junctures connected to said output circuit, each of said pairs of said discharge devices being connected to a different one of said phase windings and each discharge device including a grid for controlling the conductivity thereof, and excitation means for selectively increasing the conductivity of one pair of discharge devices to shift-I the phase the voltage applied to said output circuit supply circuit and having an intermediate con-l nection connected to said output circuit, a plurality of pairs of reversely connected velectric discharge devices having common junctures connected to said output circuit, each of` said pairs of said discharge devices being connected to a diiferent one of said phase windings and each discharge device including a grid for controlling the conductivity thereof, and excitation means connected to said grids for selectively increasing the conductivity oi.` one pair of electric discharge devices and for decreasing the conductivity of another pair of discharge devices to shift the phasev of the voltage applied to said output circuit n electrical degrees relative to the voltage of said supply circuit.

13. In combination, an alternating current supply circuit,` an output circuit, means connected between said supply circuit and said output circuit for controlling the phase of the voltage supplied to said output circuit with respect to the phase of the voltage of said supply circuit and comprising n electrically displaced phase windings connected to be energized from said supply circuit and having an intermediate connection connected to said output circuit, a plurality of pairs of reversely connected electric discharge devices having common junctures connected to said output circuit, each of said pairs of said discharge devices being connected to a different one of said phase windings and each discharge device including a grid for controlling the conductivity thereof, and excitation means connected `to the grids of the electric discharge devices for progressively controlling .the conductivities of the different pairs ofi discharge devices to shift the phase of the voltage supplied to said output circuit substantially 21- electrical degrees.

14. In combination, an alternating current supply circuit, an output circuit, means connected between said supply circuit and said output circuit for controlling the phase of the voltage supplied to said output circuit with respect to the phase of the voltage of said supply circuit and comprising n electrically displaced phase windings connected to be energized from said sup ply circuit and having an intermediate connection connected to said output circuit, a plurality of pairs of reversely connected electric discharge devices having common ,iunctures connected to said output circuit, each of said pairs of said discharge devices being connected to a diiierent one of said phase windings and each discharge device including a. grid for controlling the conductivity thereof, and excitation means connected to the grids of the electric discharge devices comprising means for selectively maintaining the conductivities oi one pair oi' discharge devices vat a constant value and selectively increasing and decreasing the conductivities of two other pairs of discharge devices to shift the phase of the voltage applied to said output circuit.

15. In combination, a supply circuit, an output circuit, means connected between said circuits and comprising a plurality of windings connected to be energized from said supply circuit and a plurality of electric discharge devices connected to said windings, said electric discharge devices each having a control member for controlling the conductivity thereof, a plurality of excitation circuits connected to the control members of the discharge devices, and a control circuit ior supplying to said excitation circuits a voltage of variable magnitude, each of said excitation circuits comprising means selectively re` sponsive to the magnitude of the voltage applied thereto by said control circuit to produce a plurality of patterns 0f control voltage as the voltage of said control circuit is varied over a given range to control selectively the conductivities of the electric discharge devices connected with said windings in accordance with the pattern of control voltage produced by one of said excitation circuits.,

16. In combination, a supply circuit, an output circuit, means connected between said supply circuit and said output circuit including a plurality of windings connected to be energized from said supply circuit and a plurality of electric discharge devices connected to said windings, a plurality of excitation circuits each connected to a different one of said discharge devices, and a control circuit common to said excitation circuits for impressing thereon a control voltage, each oi said excitation circuits comprising means selectively responsive to the magnitude of the control voltage of said control circuit to produce a different controlling action on the conductivity of the associated discharge device and thereby to effect the selective conrol of the conductivities of said discharge devices.

1'7. In combination, a supply circuit, an output circuit, means connected between said supply circuit and said output circuit and comprising a plurality of windings connected to be energized from said supply circuit and a plurality of electric discharge devices connected to said windings, each of said discharge devices being provided with a grid for controlling the conductivity thereof, a plurality of excitation circuits connected to said grids for producing control voltages and including means selectively responsive to the magnitude of the voltage applied thereto, and a control circuit common to said excitation circuits for producing a variable control voltage and cooperating with said excitation circuits for selectively controlling the conductivities of said electric discharge devices.

GEORGE L. ROGERS. 

